An objective quality assessment method of estimating subjective quality by using the physical feature amount of a video signal comprising a plurality of video frames (still images) arranged along the time axis includes a technique of estimating a subjective quality assessment value for a desired assessment target by using a reference video and a degraded video. A degraded video is a video degraded by signal processing such as encoding or network transmission with respect to a reference video, i.e., a loss in an assessment target.
In such a technique, if there is a shift in the temporal direction or spatial direction between a reference video and a degraded video, it is impossible to accurately estimate a subjective quality assessment value. It is therefore important to properly match the reference video with the degraded video.
Conventionally proposed techniques of estimating a subjective quality assessment value by using a reference video and a degraded video (see, for example, Japanese Patent Laid-Open No. 2004-080177, U.S. Pat. No. 5,446,492, and U.S. Pat. No. 6,704,451) are intended to be used for TV broadcasting, and can properly estimate subjective quality assessment values concerning accurately synchronized signals or signals at the same video size/frame rate. Therefore, these techniques are based on the assumption that a reference video and a degraded video which are matched in advance are to be used or a reference video and a degraded video which are simply matched at the start time of a video are to be used.
In video distribution services and video communication services using IP networks such as the Internet, which have recently become popular, since personal computer (PC) terminals receive videos, a reference video and a degraded video may have different sizes or different aspect ratios or a deterioration in video quality due to network performance is large. For this reason, the spatial or temporal positions of the reference video and degraded video do not match. Therefore, it is impossible to properly estimate a subjective quality assessment value by the above technique.
In order to solve this problem, the present inventors have proposed a technique of matching a reference video and a degraded video in the temporal or spatial direction and obtaining an objective assessment result by matching the signal formats of a reference video and degraded video and performing macroscopic and microscopic matching processing for these video signals for each frame (see, for example, WO2005/020592A1).